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Smart Synthesizers Simplify RF Integration

System designers stress the need for higher levels of integration in RF function blocks. Such modules provide simple digital and RF interfaces and, thus, speed the time to market and simplify system-level integration and production. Unfortunately, the RF/microwave signal sources currently available do not provide a satisfactory solution that addresses these needs. Fortunately, a line of Plug-N-Play frequency synthesizers offers a practical solution, providing a high-performance synthesized source that features quick deployment.

Currently, there are two synthesized source approaches that include (1) the integrated circuit (IC) that incorporates a phase-locked loop (PLL) and an on-board voltage-controlled oscillator (VCO) and (2) the VCO module with PLL circuitry added internally. In the first approach, the integrated VCO comes with a compromise in performance, compared to discrete, lumped-element VCO designs. In addition, most of these single-chip synthesizers require additional external circuitry, such as an external tank or loop filter. Such sources can also require significant effort in writing control software.

When discrete VCOs are integrated with the best of PLL chips and supplied in module form, the system designer is still faced with a number of problems. First, each new application calls for a custom design. Each new requirement has an impact on the frequency range, step size, reference frequency, and loop bandwidth. Once these have been established, the signal-source designer must create a unique product to fit these criteria where even the subtlest change can affect circuit values or even design topology. Additionally, the system designer is still left with the task of understanding the inner workings of the entire module prior to integration, so that control software can be developed. The risk of designing with this solution is much higher since changes during the design phase (frequency plan, step size, etc.) can be costly and time-consuming. Also, the lack of standardization in package size and interfaces leaves system designers with precious new options.

Because of the problems posed by these two synthesizer "solutions," the engineers at Universal Microwave Corp. (Odessa, FL) have developed the Plug-N-Play family (PNP series) of frequency synthesizers. These are truly configurable modules, which take mere minutes, rather than weeks, to deploy. They are designed to simplify integration for both RF designers and system software developers. The frequency synthesizers are supplied in compact surface-mount packages for ease of integration in even the smallest system designs.

These compact sources offer a host of improvements compared to traditional VCO/PLL combinations. Both I2C bus and SPI bus designers will find the PNP series straightforward with a digital interface shared by the most popular modern protocols. Code writers will find the simplicity of developing software for functional control. Any number of PNP devices can reside on the same bus, and while these modules are always on-line waiting to receive data, their internal architecture provides inherent isolation between the digital bus and the RF output.

The PNP synthesizers are extremely flexible, giving the designer the ability to configure all of the synthesizer's vital functions "on the fly," using simple strings of code that comprise the Configuration Data.

These initialization blocks of code contain commands for start frequency (START), stop frequency (STOP), frequency step size (STEP), and reference frequency (REF). Once these four variables are set, a finite number of channels are created and entering a new frequency is accomplished by updating the CHANNEL register. By delivering these simple words over the bus, the PNP architecture recognizes its new role and resets the loop to accommodate its new setup. The PNP module optimizes its internal settings for best overall integrated phase noise, switching speed, and spurious suppression, all automatically and in less than 100 µs. For example, if the system requires 100-kHz steps in one mode and 1-MHz steps in another mode, a PNP synthesizer can make the adjustments instantly without compromises in accuracy, speed, or performance.

The PNP synthesizers offer ease of integration for both RF and software engineers. The signal sources can be deployed in virtually any system without the complexity of code writing typically associated with frequency-synthesizer modules. The PNP family of intelligent frequency synthesizers can be controlled through the use of a microprocessor interface or bus. The PNP synthesizers support several protocols, such as SPI bus, Microwire interfaces, and I2C bus implementations. For SPI and Microwire applications, PNP devices require a single 32-b string of serial data to set frequency or to change internal settings (Fig. 1). The I2C bus utilizes some unique control bits and requires the addition of an Address byte, increasing the serial bit stream for this protocol to 40 b per command.

Each PNP synthesizer is programmed at the factory with presets for all of the registers. If these factory values are acceptable, there is no reason to reload any of these registers. If an application requires values other than the factory presets, the PNP synthesizer must first be initialized by loading data into each of the affected FUNCTION registers. These might include START, STOP, STEP, or REFERENCE. It is not necessary to re-load any registers that are already set properly for the application. START defines the lowest desired frequency of operation. STOP defines the highest desired frequency of operation. STEP is used to channelize the band, and REFERENCE defines the frequency of the external reference. Once the PNP synthesizer is initialized, a fixed number of channels are available. Loading the CHANNEL register sets the operating frequency of the PNP device. The formula for calculating the operating frequency is simply: START (in Hz) + = Frequency (Hz).

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Every PNP synthesizer is loaded with factory default settings so that when a module is powered up, it has a valid operating state. It then monitors the bus for instructions as to its new configuration. As an example, the model PNP-3250-L22 synthesizer has an available range of 3200 to 3300 MHz. With its factory presets, the unit will tune over the full operating band of 3200 to 3300 MHz in 1 MHz when operating with a 10-MHz external reference oscillator (Table 1).

When the step size is changed, the spurious and phase-noise performance of the PNP-3250-L22 synthesizer does not suffer, indicating that the loop in the PNP source has been reoptimized for best overall performance. When adjusting for smaller steps, the microprocessor first sends new settings to the PNP synthesizer. The only register affected by this new setup is the STEP register. As a result, a user need only send a single string of data to accomplish a move from 1-MHz steps to 100-kHz steps. However, the number of available channels increases accordingly.

The PNP frequency synthesizers can handle virtually any step size from 5 kHz to 10 MHz without degradation in phase noise. While spurious performance will obviously change with very small steps, the PNP series was designed with the goal of maintaining outstanding phase-noise performance regardless of step size. In the previous example, the PNP-3250-L22 is set for 1-MHz steps (factory default settings) and then re-programmed to take 100-kHz steps. In spite of the change, the phase-noise performance remains consistent no matter what step size is used (Fig. 2). Similarly, spurious contributions are also well controlled and consistent with changes in step size (Fig. 3).

Another example of the PNP line, the PNP-1500-P22, is designed for full-octave coverage from 1000 to 2000 MHz (Fig. 4). It achieves such wideband coverage without the use of high supply voltages typically associated with wideband frequency synthesizers. The PNP-1500-P22 is powered by supplies of +3 VDC (digital circuitry) and +12.5 VDC (analog circuitry). The compact source measures just 0.60 × 0.60 × 0.220 in. (1.5 × 1.5 × 0.56 cm) and includes the VCO, buffer amplifier, PLL, loop filter, and PNP interface within the package.

The PNP synthesizers are currently available in over 40 bands covering the frequency range from 50 MHz to 5.5 GHz (Table 2). Narrowband designs employ +3-VDC supplies for the digital circuitry and +5-VDC supplies for the analog circuitry. Wideband models are powered by +3 VDC for the digital circuitry and +12.5 VDC for the analog circuitry.

In addition to the units shown in the table, model PNP-950-L22 operates from 900 to 1000 MHz, model PNP-1250-L22 operates from 1200 to 1300 MHz, model PNP-1350-L22 runs 1300 to 1400 MHz, and model PNP-1450-L22 runs from 1400 to 1500 MHz. All these frequency synthesizers provide programmable frequency step sizes from 25 to 10,000 kHz with typical output power of 0 dBm. The phase noise for the PNP-950-L22 is typically −110 dBc/Hz offset 10 kHz from the carrier. The phase noise for the PNP-1250-L22 and PNP-1350-L22 synthesizers is −108 dBc/Hz offset 10 kHz from the carrier, and the phase noise for the PNP-1450-L22 is −107 dBc/Hz offset 10 kHz from the carrier.

The PNP-525-N22 frequency synthesizer is an example of a unit with similar frequency coverage but at much lower carrier frequencies. The PNP-525-N22 is designed for use from 50 to 1000 MHz and also can be programmed to tune in frequency steps from 25 to 10,000 kHz. While the earlier-described synthesizers are built for +5-VDC power supplies, the PNP-525-N22 operates on a +15-VDC supply (with typical current draw of 35 mA). The PNP-525-N22 achieves typical output power of 0 dBm across its full frequency range, with typical phase noise of −105 dBc/Hz offset 10 kHz from the carrier.

The compact synthesizer modules measure only 0.5 × 0.5 × 0.180 in. (1.27 × 1.27 × 0.46 cm). Any model can be preprogrammed at the factory for operation as a phase-locked oscillator (PLO) when a digital interface is not available or required. By loading a PNP source with a preset frequency of operation, a unit can be supplied for operation at any fixed frequency from 50 MHz to over 5 GHz, with only a power supply and external reference source needed for operation. The miniature synthesizers are well suited for a wide range of applications, including as precision sources in test instruments, as local oscillators (LOs) in digital microwave radios, or as compact building-block frequency sources for larger commercial or military frequency-synthesizer systems. For additional information on the programmable frequency synthesizers, please visit the company's website at www.vco1.com (specifications and outline drawings are available for more than 30 different representative models, with custom units also available).